1. Field of the Invention
The present invention relates to bismuth system oxide superconductors and the preparation thereof. More particularly, the present invention relates to bismuth system oxide superconductors comprising the 110K phase.
2. Description of the Related Art
The bismuth system oxide superconductor was found in January 1988 by Maeda et al (Jap. J. Appl. Phys., 27, (1988) L209). With this bismuth system oxide, a sharp decrease of electrical resistance was observed around 110K and also the Meissner effect was observed in the same temperature range. Since the critical temperature of 110K is much higher than 90K of YBa.sub.2 Cu.sub.3 O.sub.y, which had been believed to have the highest critical temperature, the bismuth system oxide is expected to find various applications in future.
It is known that a phase of the bismuth system oxide having the critical temperature of 110K can be isolated as a single crystal. However, the isolated single crystal cannot be used as a superconductive material. Thus, it can be said that at the time of finding by Maeda et al, no practically usable material consisting of a single phase of the bismuth system oxide having the critical temperature of 110K was produced, and the produced material contained impurities such as a 80K phase or a semiconductive phase and Ca.sub.2 PbO.sub.4. In addition, the X-ray diffraction pattern of the bismuth system oxide had weak diffraction peaks other than that assigned to the 110K phase, although compounds to which such weak diffraction peaks are assigned were not specified. Since the presence of several phases in the material seriously deteriorates the superconductive properties, it was desired to provide a superconductive material essentially consisting of the 110K phase.
To increase the proportion of the 110K phase in the bismuth system oxide, it was proposed to heat said oxide. However, the heating took long time. For example, it took several days at 880.degree. C. in the air (presentation in the International Conference on High Temperature Superconductors and Material and Mechanisms of Superconductivity held on Feb. 29 to Mar. 4, 1989 in Interlaken, Switzerland), or eleven days in the air (90th Annual Meeting of the American Ceramics Society held on May 1 to 5, 1988 in Cincinnati, Ohio, U.S.A.). Some other measures to increase the proportion of the 110K phase were reported. For example, Takano et al proposed the addition of lead (Pb), and Kijima et al proposed the variation of the composition. Although they achieved some improvements, their process requires a long reaction time, for example, 244 hours in the air by Takano et al or 120 hours in the air by Kijima et al.
In the practical production of the oxide superconductive materials, the decrease of the reaction temperature is one of the important requirements. In the field of semiconductors, the oxide superconductor should be produced at a sintering temperature as low as possible so as not to deteriorate properties of silicon substrates. In a case of Y-system and Tl-system oxides, the bulk materials should be sintered at about 900.degree. C. and 1,000.degree. C., respectively. Although the bismuth system oxide can be sintered at a comparatively low temperature, the sintering temperature is still about 870.degree. C.